The use of a fluidized bed incinerator for disposing of refuse such as municipal wastes is known. The method for incinerating and disposing the refuse in such fluidized bed incinerator comprises burning the refuse with air while fluidizing it in a fluidized bed. In order to improve the combustion along with the fluidizing of refuse, a fluidizing medium such as sand is fed together with the refuse to the fluidized bed.
A typical type of fluidized bed incinerator is equipped with a plurality of air diffuser tubes or plates in the lower section of the incinerator body, and the upper section of the incinerator body is equipped with a refuse feeding unit and a fluidizing medium feeding unit.
The refuse is burned while both the refuse and the fluidizing medium deposited into the air diffusers inside the incinerator body are fluidized by primary air which is blown out through the air diffusers.
Refuse in the form of municipal waste generally contains a variety of materials such as low calorie refuse, e.g. food discards, high calorie refuse such as plastics and rubber, refuse such as shredded paper and chipped furniture, or refuse such as fragmented metallic or vitreous containers, bottles, or cans.
As the refuse is fed to the fluidized bed, the combustibles are burned, of which the plastics and similar substances are melted by heat to generate pyrolysis gases, and the incombustible substances like glass are left unburned (called "combustion residue" hereinafter).
The fluidizing medium descends as it is gradually fed to the fluidized bed. Consequently, while the combustibles are burned and decomposed within the fluidizing bed, the incombustibles descend along with the fluidizing medium down through the incinerator and pass through the gaps among air diffuser tubes in the lower section of the fluidized bed. The fluidizing medium is separated from combustion residue, and is returned to the fluidized bed.
Secondary air is fed to an upper section of the incinerator above the fluidized bed in order to burn the pyrolysis gases.
Because the fluidizing medium fed to the fluidized bed is heated and oscillated while it descends, agitation and dispersion of the refuse is promoted. As a result, the refuse deposited onto the fluidized bed is dispersed uniformly through the fluidizing medium, and is therefore dried, ignited, decomposed, and burned instantly. Further, the resulting ashes and dust are carried by the fluidizing air out of the upper section of the incinerator and are collected by an electric precipitator.
Consequently, the refuse deposited onto the fluidized bed is disposed almost completely, leaving behind some metallic, vitreous, or ceramic residue. The ratio of these latter-mentioned substances to the refuse is usually 2%, thus incineration by a fluidized bed incinerator can dispose of 98% of the refuse. In fact, one of the advantages of the fluidized incinerator is that the volume of combustion residue can be reduced to one-third compared with a conventional mechanical incinerator such as a stoker-type incinerator.
Because the refuse deposited onto the fluidized bed is burned and decomposed rapidly, the refuse cannot be burned stably. The refuse has different calorific value depending on the content of the refuse, and it is often difficult to consistently supply a constant volume of refuse to the fluidized bed. For example, if a large quantity of refuse is introduced to the fluidized bed at once, a large quantity of pyrolysis gases and smut are also generated simultaneously, and the refuse undergoes pyrolysis instantaneously. Under these conditions, not only is it impossible to completely combust a large quantity of pyrolysis gas with the secondary air inside the incinerator, but it is also difficult to collect all of the large quantity of smut in exhaust gas by means of an electric precipitator.